The present invention generally relates to an apparatus and a method for determining corrosion rate. More specifically, the invention relates to a transient technique utilizing a potential step or pulse and applying that signal to a metal/solution interface in order to determine solution resistance. A measurement of corrosion rate can be ascertained from at least the solution resistance.
A number of methods are known for ascertaining the rate of corrosion. Typically, electrochemical techniques for corrosion measurements are based on the Stearn-Geary equation. This equation is generally known as follows: ##EQU1## wherein I.sub.corr =corrosion rate (current density),
B.sub.a =anodic Tafel slope (mV), PA1 B.sub.c =cathodic Tafel slope (mV), and PA1 R.sub.p =polarization resistance (Ohms.cm.sup.2)
From this equation, the corrosion rate can be determined by measuring the polarization resistance R.sub.p for known values of B.sub.a and B.sub.c. The corrosion rate in current density, I.sub.corr, can further be converted to corrosion rates in other units, such as mils per year (mpy) and millimeters per year (mm/yr.), based on Faraday's law.
In a conventional linear polarization technique, a small potential excitation is applied across a corroding metal surface at a very slow scan rate to quantify the total resistance as the sum of solution resistance (R.sub.s) and polarization resistance (R.sub.p). Therefore, linear polarization can be used for corrosion rate measurements when R.sub.s is very small compared to R.sub.p wherein EQU R.sub.s +R.sub.p .apprxeq.R.sub.p.
However, in many instances, R.sub.s is not negligible as compared to R.sub.p. Therefore, R.sub.s must be determined in a separate experiment. That is, to accurately measure the corrosion rate, a conventional linear polarization technique cannot be implemented if R.sub.s is not negligible.
One known method for calculating solution resistance (R.sub.s) is known as the "current interrupt" method. In this method, a small current i.sub.c is applied to an electrode for a short time period. At a later point in time, the current i.sub.c is interrupted. The potential difference between this electrode and a reference electrode is monitored before and after the current i.sub.c is interrupted. FIG. 7 illustrates the manner in which the solution resistance R.sub.s is estimated using this method by plotting the potential as it varies over time. That is, from the measurements, the solution resistance R.sub.s may be approximated or extrapolated by the equation: ##EQU2## The actual solution resistance is E.sub.R /i.sub.c as indicated in FIG. 7. This current interrupt method, however, provides an approximation or extrapolation that can be very inaccurate for calculating solution resistance, R.sub.s, especially when R.sub.s is large.
Another method for determining the solution resistance R.sub.s is known as the "AC impedance" technique or "Electrochemical Impedance Spectroscopy (EIS)." With this method, the actual solution resistance may be calculated from the equation: ##EQU3## wherein a small amplitude AC sinusoidal potential wave, .DELTA.E, at a very high frequency (freq.), typically 10 KHz, is applied to the metal surface in the solution to measure the current response, .DELTA.i.
The apparatus required, however, for performing calculation of the solution resistance using the AC impedance method (or Electrochemical Impedance Spectroscopy, EIS) is expensive and furthermore is not well-suited for long term field use due to its size and weight.
A need, therefore, exists for an improved method for determining solution resistance to provide simple and accurate corrosion rate measurements.